Blood-clotting proteins, interaction

Vitamin K status can be assessed by a functional test, called the "prothrombin time test," which involves measuring the lime required to form a blood clot. The test is performed as follows. A blood sample is withdrawn from a subject and immediately mixed with citric acid. Citric acid is a chelator, which means that it can form a tight complex with ions, such as calcium ions. The chelator prevents the interaction of calcium ions with the blood-clotting proteins and thus prevents these proteins from forming a blood clot in the sample. Calcium ions, it should be noted, are required for supporting the activity of several blood clotting proteins. The "citrated blood" is placed in a machine called a fibrometer. The fibrometer is used to detect increases in the viscosity of the blood over a period. 

This study deals with the formation of complexes between blood clotting proteins and natural and artificial surfaces. As these surfaces are generally charged, the behavior of a basic protein, cardiotoxin (CTX), the interaction of which is strictly charge-dependent, is also reported for comparison. Two types of interface have been investigated. 

Surfaces can be active in inducing blood clotting, and there is much current searching for thromboresistant synthetic materials for use in surgical repair of blood vessels (see Ref. 111). It may be important that a protective protein film be strongly adsorbed [112]. The role of water structure in cell-wall interactions may be quite important as well [113]. 

A conformational change induced in the protein antithrombin (AT) on binding a specific pentasaccharide S domain allows its interaction with Factor Xa, a blood clotting factor, preventing clotting. 

Binding of AT and thrombin to two adjacent S domains brings the two proteins into close proximity, favoring their interaction, which inhibits blood clotting. 

At least three other families of plasma membrane proteins are also involved in surface adhesion (Fig. 11-22). Cadherins undergo homophilic ( with same kind ) interactions with identical cadherins in an adjacent cell. Immunoglobulin-like proteins can undergo either homophilic interactions with their identical counterparts on another cell or heterophilic interactions with an integrin on a neighboring cell. Selectins have extracellular domains that, in the presence of Ca2+, bind specific polysaccharides on the surface of an adjacent cell. Selectins are present primarily in the various types of blood cells and in the endothelial cells that line blood vessels (see Fig. 7-33). They are an essential part of the blood-clotting process. 

The induction of a change in one protein by interaction with another protein is a phenomenon that is met also in the construction of microtubules, ribosomes, cilia, and myofibrillar assemblies of muscle. It is basic to the assembly of the many labile but equally real cascade systems of protein-protein interactions such as that involved in the clotting of blood (Chapter 12) and signaling at membrane surfaces. 

If thrombin and factor Xa, the major activated blood coagulation factors (Fig. 11.6), escape into healthy blood vessels, blood clots will develop and occlude capillaries throughout the body. Direct inhibition of these activated enzymes in the blood flow utilizes serine protease inhibitors, of which there are two common types a Kunitz inhibitor and a serpin. The former possess a Kunitz domain, a convex antiparallel (1-sheet that exactly fits into the concave active site of a serine protease, directly blocking it (lock and key mechanism). By contrast, serpins undergo complex interactions with other proteins to cause conformational changes that bait and block the catalytic action (Fig. 11.12 shows the bait). Table 11.3 fists the major coagulation inhibitors and cofactors, their targets and mechanisms of action. 

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